Known injectors of the above type comprise a hollow body for housing the metering valve body; the valve body has a calibrated hole for discharging high-pressure fuel from an injection control chamber; the calibrated hole is kept closed by a normally disk-type shutter controlled by the armature of a control electromagnet, and normally comes out at a conical depression in a surface of the valve body; the shutter comprises a ball cooperating with a plate engaged by the armature and having a spherical-bowl-shaped recess to eliminate any misalignment between the hole axis and displacement of the armature; and the valve body is locked inside a cavity in the hollow body by a ring nut, which requires a given distance between the electromagnet and the valve body.
Known injectors of this type have several drawbacks. In particular, the distance between the valve body and the electromagnet means the disk-type armature must be provided with a corresponding stem to act on the plate; the armature stem, in turn, calls for high-cost guide means, normally precision-machined sleeves; machining the valve body with the conical depression is also relatively expensive; and, finally, the conical depression increases the necessary length of the armature stem.
In one known injector, a valve body is proposed in which the calibrated hole comes out at a flat surface of the valve body, and is closed by a flat surface of a plate, another surface of which may also be flat and is engaged by a convex end of the armature stem. In an alternative solution, the other surface of the plate has a spherical-bowl-shaped recess engaged by a complementary appendix on the stem. This known injector does not eliminate the need for an armature stem, and, what is more, makes the armature stem even more expensive.
Another injector has recently been proposed, in which the valve body is defined by a sleeve force-fitted inside a cavity in the hollow body; and the disk-type armature is connected to the hollow body by a leaf spring hinge having one portion fixed to the hollow body, and one portion fixed to the armature. This reduces the distance between the valve body and the electromagnet, but, on account of the conical surface of the valve body at which the calibrated hole comes out, the disk-type armature, to act on the plate, calls for a spacer member normally carried by the armature.
This type of injector is also relatively expensive, on account of the spacer member and relative connection. Moreover, on account of the leaf spring hinge, the armature movement is not perfectly straight, so that the contact surfaces between the spacer member on the armature and the shutter plate are subject to relative transverse slippage, which reduces the sensitivity and reliability of the valve, and which increases alongside an increase in the distance of the point at which the spacer member engages the plate.